A DRAM is a semiconductor device for storing digital information. Data, as digital information, can be written to and read from a DRAM. DRAMS are fabricated using integrated circuit technology.
A DRAM is made of many storage nodes or memory cells and each memory cell has a memory cell transistor and a capacitor. The capacitor is an important element of the memory cell because it stores the digital information. Trench capacitors and stack capacitors are the two major types of DRAM cell capacitors.
The higher the capacitance of the capacitor the better. Higher capacitance improves the data sensing margin and increases the shrinkability of the memory cell. One way to increase capacitance is to increase the surface area of the capacitor (capacitor area). The trench capacitor increases the capacitor area by digging a deep trench inside the bulk silicon. The stack capacitor increases the capacitor area by raising the height of the capacitor above the surface of the silicon wafer. One particular type of stack capacitor has a cylindrical shape to increase the capacitor area without increasing the area and height of the memory cell. Stack capacitors are also referred to as capacitor-over-bit-line (COB) capacitors.
DRAM density is ever-increasing and the size of memory cell area on the DRAM chip is decreasing. Each memory cell capacitor uses a large amount of space on the DRAM. The need for memory cell capacitors to be isolated from each other also uses space. To increase DRAM density, a method is needed to further increase the size of the DRAM memory cell capacitor and to decrease the spacing between memory cell capacitors.